1. Field of the Invention
This invention relates to a phase-to-sinusoid amplitude convert (PSAC) of the direct digital frequency synthesizer, particularly to a convert circuit of the direct digital frequency synthesizer using Pade approximation algorithm for converting phase into a quarter period of sinusoidal wave signal.
2. Brief Description of the Prior Art
Generally, a phase accumulator 10′ and a phase-to-sinusoid amplitude converter 20′ are used in the so-called direct digital frequency synthesis (hereinafter expressed as DDFS) to generate required digitized signal, and then a digital to analog converter (DAC) is used to convert the digitized signal into analog waveform. In FIG. 1, a conventional direct digital frequency synthesizer is shown, which includes a phase accumulator 10′, a phase-to-sinusoid amplitude converter 20′ and a DAC 30′. The phase accumulator 10′ of the direct digital frequency synthesizer receives a digital signal and outputs it to the phase-to-sinusoid amplitude converter 20′. Thus an corresponding output signal is generated. The digital signal is converted into analog signal by the DAC 30′. The analog signal served as the output of the DAC 30′ can be smoothed by a low pass filter (not shown) so as to generate a periodic signal such as a sinusoidal wave.
Inasmuch as the DDFS has the advantages of high resolution, speedy frequency switching, continuous linear variation of phase, low cost and simple structure, DDFS has been widespread utilized in digital communication system. So far, Taylor polynomial and CORDIC algorithm are used as the direct calculation algorithm of the DDFS. It is relatively easy to realize the DDFS by sinusoidal approximation calculation with direct expanding of Taylor polynomial. The phase input is operated by continuous multiplication calculation and sinusoidal wave symmetry property configuration to generate a complete sinusoidal wave signal. Further, CORDIC algorithm is a method to calculate the sine and cosine values corresponding to the input phase by the trigonometric function property, in which the sine and cosine values corresponding to all rotation phases after addition and subtraction are calculated by multiplication, addition and shift. In addition, using the property of fixing the rotation angle at arctan(2−r) all the time, a multiplier is replaced by a shifter. Thus a complete sinusoidal wave signal is generated by the sinusoidal wave symmetry property.
However, in the sinusoidal wave approximation calculation by direct expanding of Taylor polynomial, realization of a quarter period of a sine wave from the phase input has to calculate eight-time continuous multiplication. Thus, the calculation is relatively time-consuming and the circuit area required for this calculation is large. Furthermore, the frequency of rotation iterative calculation is influenced by the selection of initial angle and rotation angle in CORDIC algorithm, thus the calculation of corresponding amplitude according to the input angle is relatively time-consuming and the frequency of rotation iterative calculation is too high. Therefore, the realization of a complete sinusoidal wave is relatively time-consuming.
Thus, if a new direct digital frequency synthesizer is proposed to cope with the above problems, not only the time for the calculation of a complete sinusoidal wave can be saved, but also the circuit area can be reduced so as to save the cost. Accordingly, the above problems can be solved.